1. Field of the Invention
This invention relates generally to battery operable ambulatory and non-ambulatory patient monitoring systems and in particular to an ambulatory patient monitoring device that includes storage to a solid-state flash memory which storage is controlled in a manner to optimize power consumption.
2. Description of Related Art
Ambulatory monitoring of patient data is well known in the art and is useful in monitoring the health of the patient. There are many types of patient physiological parameters that need to be monitored for particular medical examinations. The most prominent type of ambulatory monitoring is ECG monitoring. Such ambulatory ECG monitoring systems are also known as Holter monitors. The ECG data is acquired by the Holter monitor continuously over a fixed period, generally twenty-four hours. Such devices are described in U.S. Pat. Nos. 5,228,450; 5,205,295; and 5,027,824, each of which is incorporated herein by reference in its entirety.
While the Holter monitor is a well-known ambulatory device for ECG measurements, other ambulatory devices have been developed for monitoring brain wave functions and are known as ambulatory EEG devices. U.S. Pat. No. 5,222,503 discloses a history of such devices and is incorporated herein by reference in its entirety.
It has also been discovered that in the field of sleep disorders, a correct diagnosis should include details of the ECG, the EEG, the EMG (muscle measurement), EOG (rapid eye movement or REM), respiratory activities such as breathing or airflow, respiratory physical response such as movement of the chest or stomach, and blood oxygen saturation. To be effective, all of these different physiological parameters must be analyzed simultaneously. In the analysis of various physiological conditions, it may be extremely important to correlate the ECG, the EEG, the EMG, the EOG, the respiratory activity, the respiratory mechanics, and blood oxygen saturation. At present, it is impossible to provide such correlation using a portable battery operable device. A sleep monitoring device is disclosed in U.S. Pat. No. 5,187,657, incorporated herein by reference in its entirety.
Conventional ambulatory ECG Holter monitors have fallen into two categories: cassette tape-based systems and solid-state systems. They are worn by the patient outside the hospital during the patient's normal daily routine. Tape based systems comprise a magnetic tape recorder that records ECG signals on to the cassette tape from electrodes that are attached to the patient in a predefined fashion. On completion, the tape is removed from the recorder and the ECG data stored on the cassette tape are analyzed usually on a personal computer-based scanning system.
There are numerous problems with the tape-based systems. Primarily, the cassette tapes have a limited frequency response. The maximum frequency response of such tapes is around 40 Hz. But, conventional diagnostic ECG systems require a frequency range from 0.1 Hz to 100 Hz. Further, the latest high resolution EGG analysis require that data should be recorded to at least 300 Hz. Moreover, for high resolution ECG devices, the devices should have the capability to record a minimum of 1000 samples per second. Such high fidelity recording is not possible on tape-based systems. The highest frequency response of the magnetic tape devices is further limited by the small recording range on the tape. In addition, tape-based systems suffer from motion error problems including speed changes and recording head tracking errors. These problems are further compounded by the recorder's inability to precisely encode important events such as abnormal heart beats and pacemaker spikes on the tapes.
Solid-state systems consist of solid-state memory that digitally stores the ECG data. ECG data are recorded and stored to this memory for future analysis. In solid-state systems, in order to store the enormous amounts of data, the prior art units have relied on severe data compression, with a resultant distortion of the data. Typically, such devices have a memory capacity of 2-to-4 megabytes when, in fact, they should have in the neighborhood of 30-to-80 megabytes. Because the data must be compressed enormously for memory capacity, serious loss of data occurs. In U.S. Pat. No. 5,222,503, the memory device is a separate unit worn about the waist of the patient. Also, because of the amount of data that must be stored, power requirements for the units are significantly increased. Further, they use typically volatile memories so that the data held in the memory is lost upon an interruption of power which is not uncommon in battery operated devices. Recently, an ambulatory ECG monitor was introduced which incorporated a miniature hard disk drive for data storage. This electromechanical design is covered by U.S. Pat. No. 5,228,450. Systems that employ non-volatile memory in the form of miniature hard disk drives for mass data storage have moving parts that may damage the data in harsh environments typical to Holter monitoring. Further, systems that use non-volatile memory units such as PCMCIA memory cards do not have the cards built into the monitor. Instead, they are attached to the monitor through a connector. In such systems, the memory card may get disconnected in the middle of a recording. Furthermore, such cards, unlike cassette tapes, are very expensive and might be easily lost or misplaced.
A recent article in The American Journal of Cardiology, Vol. 68, Oct. 15, 1991, entitled "Combined Ambulatory Electroencephalographic and Electrocardiographic Recordings for Evaluation of Syncope", presented good clinical results for a subset of the syncope (fainting or light-headedness) population. In the study, both an ambulatory 2 channel ECG recording system and an ambulatory EEG recording system (including 7 channels of EEG and 1 channel of ECG) were connected to the patients for 24 hours with two separate ambulatory monitors, one for ECG and one for EEG. The present innovative design would allow this clinical information (i.e., both cardiology and neurology workups) with one study because only one ambulatory monitor would be used.
It would be advantageous to provide an ambulatory or portable device for storing patient physiological data to be used for subsequent medical diagnosis in which the device could monitor a plurality of channels having data representing patient physiological data such as ECG, EEG, EMG, EOG, respiratory activities, respiratory mechanics, and blood oxygen saturation simultaneously or in such combinations as needed.
Further, it would be advantageous to have such an ambulatory monitoring device in which a plurality of input channels have analog physiological data thereon and in which one or more channels of one or more of said physiological parameters such as 2 or 3 channels of ECG and 7 channels of EEG for a syncope study could be selectively accessed for recording.
Further, it would also be advantageous to have such an ambulatory monitoring device in which a plurality of input channels having analog physiological data can be converted to digital samples on each separate one of the channels selected at an individually selectable sampling rate for converting the raw analog data in each channel to raw digital data for recording.
It would also be advantageous to have a battery-operated ambulatory monitoring device which had a high capacity flash memory for storing large amounts of data but which could be controlled by a flash memory controller applied specifically to storing both raw data and appropriate measurement data in the flash memory with optimum power usage.
It would be further advantageous to have such an ambulatory monitoring device that employs flash memory built into the device for mass storage of a plurality of input signals that does not suffer from the power consumption and other problems of tape-based systems, solid-state systems, and other systems using the hard disk drives and PCMCIA memory cards.